The present invention relates broadly to gaskets for providing environmental sealing and/or electromagnetic interference (EMI) shielding, and particularly to a low closure force EMI shielding gasket which is especially adapted for use within small electronics enclosures such as cellular phone handsets and other handheld electronic devices.
The operation of electronic devices such as televisions, radios, computers, medical instruments, business machines, communications equipment, and the like is attended by the generation of electromagnetic radiation within the electronic circuitry of the equipment. As is detailed in U.S. Pat. Nos. 5,202,536; 5,142,101; 5,105,056; and 4,857,668, such radiation often develops as a field or as transients within the radio frequency band of the electromagnetic spectrum, i.e., between about 10 KHz and 10 GHz, and is termed xe2x80x9celectromagnetic interferencexe2x80x9d or xe2x80x9cEMIxe2x80x9d as being known to interfere with the operation of other proximate electronic devices.
To attenuate EMI effects, shielding having the capability of absorbing and/or reflecting EMI energy may be employed both to confine the EMI energy within a source device, and to insulate that device or other xe2x80x9ctargetxe2x80x9d devices from other source devices. Such shielding is provided as a barrier which is inserted between the source and the other devices, and typically is configured as an electrically conductive and grounded housing which encloses the device. As the circuitry of the device generally must remain accessible for servicing or the like, most housings are provided with openable or removable accesses such as doors, hatches, panels, or covers. Between even the flattest of these accesses and its corresponding mating or faying surface, however, there may be present gaps which reduce the efficiency of the shielding by presenting openings through which radiant energy may leak or otherwise pass into or out of the device. Moreover, such gaps represent discontinuities in the surface and ground conductivity of the housing or other shielding, and may even generate a secondary source of EMI radiation by functioning as a form of slot antenna. In this regard, bulk or surface currents induced within the housing develop voltage gradients across any interface gaps in the shielding, which gaps thereby function as antennas which radiate EMI noise. In general, the amplitude of the noise is proportional to the gap length, with the width of the gap having less appreciable effect.
For filling gaps within mating surfaces of housings and other EMI shielding structures, gaskets and other seals have been proposed both for maintaining electrical continuity across the structure, and for excluding from the interior of the device such contaminates as moisture and dust. Such seals are bonded or mechanically attached to, or press-fit into, one of the mating surfaces, and function to close any interface gaps to establish a continuous conductive path thereacross by conforming under an applied pressure to irregularities between the surfaces. Accordingly, seals intended for EMI shielding applications are specified to be of a construction which not only provides electrical surface conductivity even while under compression, but which also has a resiliency allowing the seals to conform to the size of the gap. The seals additionally must be wear resistant, economical to manufacture, and capability of withstanding repeated compression and relaxation cycles. For further information on specifications for EMI shielding gaskets, reference may be had to Severinsen, J., xe2x80x9cGaskets That Block EMI,xe2x80x9d Machine Design, Vol. 47, No. 19, pp. 74-77 (Aug. 7, 1975).
EMI shielding gaskets typically are constructed as a resilient core element having gap-filling capabilities which is either filled, sheathed, or coated with an electrically conductive element. The resilient core element, which may be foamed or unfoamed, solid or tubular, typically is formed of an elastomeric thermoplastic material such as polyethylene, polypropylene, polyvinyl chloride, or a polypropylene-EPDM blend, or a thermoplastic or thermosetting rubber such as a butadiene, styrene-butadiene, nitrile, chlorosulfonate, neoprene, urethane, or silicone rubber.
Conductive materials for the filler, sheathing, or coating include metal or metal-plated particles, fabrics, meshes, and fibers. Preferred metals include copper, nickel, silver, aluminum, tin or an alloy such as Monel, with preferred fibers and fabrics including natural or synthetic fibers such as cotton, wool, silk, cellulose, polyester, polyamide, nylon, polyimide. Alternatively, other conductive particles and fibers such as carbon, graphite, or a conductive polymer material may be substituted.
Conventional manufacturing processes for EMI shielding gaskets include extrusion, molding, or die-cutting, with molding or die-cutting heretofore being preferred for particularly small or complex shielding configurations. In this regard, die-cutting involves the forming of the gasket from a cured sheet of an electrically-conductive elastomer which is cut or stamped using a die or the like into the desired configuration. Molding, in turn, involves the compression or injection molding of an uncured or thermoplastic elastomer into the desired configuration.
More recently, a form-in-place (FIP) process has been proposed for the manufacture of EMI shielding gaskets. As is described in commonly-assigned U.S. Pat. Nos. 6,096,413; 5,910,524; and 5,641,438, and PCT Application WO 96/22672; and in U.S. Pat. Nos. 5,882,729 and 5,731,541; and Japanese Patent Publication (Kokai) No. 7177/1993, such process involves the application of a bead of a viscous, curable, electrically-conductive composition which is dispensed in a fluent state from a nozzle directly onto to a surface of a substrate such as a housing or other enclosure. The composition, typically a silver-filled or otherwise electrically-conductive silicone elastomer, then is cured-in-place via the application of heat or with atmospheric moisture or ultraviolet (UV) radiation to form an electrically-conductive, elastomeric EMI shielding gasket in situ on the substrate surface.
Another recent EMI shielding solution for electronics enclosures, which solution is further described in commonly-assigned U.S. Pat. No. 5,566,055 and in DE 19728839 involves the over-molding of the housing or cover with an conductive elastomer. The elastomer is integrally molded in a relatively thin layer across the inside surface of the housing or cover, and in a relatively thicker layer along the interface locations thereof providing both a gasket-like response for environmentally sealing the cover to the housing and electrical continuity for the EMI shielding of the enclosure. The elastomer additionally may be molded onto interior partitions of the cover or housing, or itself molded to integrally-form such partitions, providing electromagneticly-isolated compartments between potentially interfering circuitry components. Covers of such type are marketed commercially under the name Cho-Shield(copyright) Cover by the Chomerics EMC Division of Parker-Hannifin Corporation (Woburn, Mass.).
Yet another solution for shielding electronics enclosures, and particularly the smaller enclosures typical of cellular phone handsets and other handheld electronic devices, concerns the incorporation of a thin plastic retainer or frame as a supporting member of the gasket. As is described in commonly-assigned U.S. Pat. No. 6,121,545, the electrically conductive elastomer may be molded or, as is described in U.S. Pat. No. 5,731,541, formed-in-place or otherwise attached to the inner or inner peripheral edge surfaces and/or to the upper or lower face surfaces of the frame. So constructed, the gasket and frame assembly may be integrated within the electronic device to provide a low impedance pathway between, for example, peripheral ground traces on a printed circuit board (PCB) of the device, and other components thereof such as the conductive coating of a plastic housing, another PCB, or a keypad assembly. Uses for the spacer gaskets of the type herein include EMI shielding applications within digital cellular, handyphone, and personal communications services (PCS) handsets, PC cards (PCMCIA cards), global positioning systems (GPS), radio receivers, and other handheld devices such as personal digital assistants (PDAs). Other uses include as replacements for metal EMI shielding xe2x80x9cfencesxe2x80x9d on PCBs in wireless telecommunication devices.
Requirements for typical small enclosure applications generally specify a low impedance, low profile connection which is deflectable under relatively low closure force loads, e.g., about 1.0-8.0 lbs per inch (0.2-1.5 kg per cm) of gasket length. Usually, a minimum deflection, typically of about 10%, also is specified to ensure that the gasket sufficiently conforms to the mating housing or board surfaces to develop an electrically conductive pathway therebetween. It has been observed that for certain applications, however, that the closure or other deflection force required to effect the specified minimum deflection of conventional profiles may be higher than can be accommodated by the particular housing or board assembly design.
One method of achieving a lower closure force gasket design particularly adapted for use in smaller electronic enclosure packages has been to form the gasket as having a periodic xe2x80x9cinterruptedxe2x80x9d pattern of alternating local maxima and minima heights. Conventionally, and as is described in commonly-assigned, co-pending application U.S. Ser. No. 09/703,240, filed Oct. 31, 2000, in the Technical Publication xe2x80x9cEMI Shielding and Grounding Spacer Gasket,xe2x80x9d Parker Chomerics Division, Woburn, Mass. (1996), and in PCT application 98/54942, gaskets of such type may be formed by molding or the FIP process as having a crenelated, i.e., notched, serrated, or a sinusoidal xe2x80x9cwaveformxe2x80x9d profile, or as a series of discrete beads. In general, for a specified joint configuration, a gasket having such an xe2x80x9cinterruptedxe2x80x9d profile or pattern would be expected to exhibit a greater deflection under a given compressive load than a continuous profile.
Another method of achieving a lower closure force in a spacer gasket design is described in commonly-assigned U.S. Pat. No. 6,121,545. Such method involves configuring the gasket as having a moment arm portion which is angularly deflectable in an inward or outward direction relative to the frame responsive to a compressively-applied load. As a result of the described bending mode response, such gasket is seen to exhibit force deflection than gasket profiles operating in a conventional compression mode.
As the sizes of handheld electronic devices such as cellular phone handsets has continued to shrink, it will be appreciated that further improvements in the design of gaskets profiles therefor would be well-received by the electronics industry. Especially desired would be a low closure force gasket profile which is adapted for use in the smaller electronics enclosures which are increasingly becoming the industry standard.
The present invention is directed to a low closure force gasket profile for environmental sealing and/or electromagnetic interference (EMI) shielding which is especially adapted for use in smaller electronic enclosure packages. The gasket profile of the invention features a moment arm or bending portion which is responsive to a compressively-applied load, such as when compressed between two surfaces of a housing or other enclosure, to be angularly deflectable in an inward or outward direction relative to the interior or exterior of the housing or other substrate on which the gasket may be supported. The described bending mode response advantageously provides a large but controlled deflection affording a more uniform interface contact with the contacting surface for more assured electrical and physical continuity and, in turn, more reliable EMI shielding and environmental sealing effectiveness. When employed, for example, in electronics applications, the gasket of the invention therefore provides consistent EMI shielding and, additionally, environmental sealing effectiveness.
In accordance with the precepts of the present invention, the bending mode response of the gasket is controlled by the incorporation of a stress relief portion on the side of the gasket opposite the side to which the gasket is deflected about a bending point. In this regard, it has been observed though mathematical modeling that the gasket side opposite the bending point develops a local region of high stress or strain which develops as the side is stretched. By forming a shoulder, notch, or other stress relieving feature into this region of high stress, the force required for the transition of the gasket from a compression to a bending mode response may be lowered with a corresponding decrease in the load force required for a given deflection. The result is a gasket profile which is observed to exhibit lower closure force requirements as compared to a gasket profile which operates solely in a compression rather than a bending mode or which lacks the described stress relief feature. That is, for a specified joint configuration, the gasket profile of the present invention exhibits a greater deflection under a given compressive load than more conventional profiles.
In an illustrative embodiment, the gasket profile of the invention may be molded or otherwise supported on an edge or other surface of a housing or other enclosure for an electronic device such as a cellular telephone. When supported on an inward edge surface of the enclosure, the moment arm portion of the gasket may be angled or otherwise shaped to develop an outward force component advantageously directing the deflection of the moment arm portion toward the enclosure edge and away from the interior thereof wherein the gasket might otherwise interfere with the proper operation of the circuits or other electronics being housed within the enclosure.
Advantages of the present invention include the provision of an improved gasket profile for low closure force applications such as may be found in small, handheld electronic devices. Additional advantages includes a gasket profile which exhibits a controlled deflection response for more stable interface contact with the housing or circuit board components of the enclosure and, in turn, more assured electrical continuity and reliable EMI shielding effectiveness. These and other advantages will be readily apparent to those skilled in the art based upon the disclosure contained herein.